Op-Amp Applications
Scientech 2323
Product Tutorial
Ver. 1.1
Designed & Manufactured byAn ISO 9001:2008 company
Scientech Technologies Pvt. Ltd.
94, Electronic Complex, Pardesipura, Indore - 452 010 India,
+ 91-731 4211100, : info@scientech.bz , : www.ScientechWolrd.com
Scientech 2323
Op-Amp Applications
Scientech 2323
Table of Contents
1.
2.
3.
4.
5.
6.
Safety Instructions
Introduction
Features
Technical Specifications
Theory
Experiments
•
Experiment 1
To study and observe Op-Amp as Voltage Comparator
•
Experiment 2
To study and observe Op-Amp as Zero Crossing Detector
•
Experiment 3
To observe the Op-Amp working as Logarithmic Amplifier
•
Experiment 4
To observe the Op-Amp working as antilogarithmic Amplifier
•
Experiment 5
To study and observe Op-Amp as a Peak Detector
•
7.
Experiment 6
To study and observe Op-Amp as a Wien Bridge Oscillator and its
gain factor for a smooth sine wave
•
Experiment 7
To study and observe Op-Amp as a Phase Shift Oscillator and its
phase shift at every RC combination
•
Experiment 8
To study and observe Op-Amp as a Function generator, generating
Square and Triangle wave
•
Experiment 9
To study and observe Op-Amp as a Half Wave Precision Rectifier
•
Experiment 10
To study and observe Op-Amp as active second order High Pass
Filter
•
Experiment 11
To study and observe Op-Amp working as active second order
Low Pass Filter
•
Experiment 12
To study and observe Op-Amp working as active second order
Band Pass Filter
•
Experiment 13
To study and observe Op-Amp working as active Notch Filter
Warranty & List of Accessories
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Safety Instructions
Read the following safety instructions carefully before operating the product.
To avoid any personal injury, or damage to the product, or any products connected to
it;
Do not operate the instrument if you suspect any damage within.
The instrument should be serviced by qualified personnel only.
For your Safety:
Use proper Mains cord
: Use only the mains cord designed for this product.
Ensure that the mains cord is suitable for your
country.
Ground the Instrument
: This product is grounded through the protective earth
conductor of the mains cord. To avoid electric shock
the grounding conductor must be connected to the
earth ground. Before making connections to the input
terminals, ensure that the instrument is properly
grounded.
Observe Terminal Ratings : To avoid fire or shock hazards, observe all ratings and
marks on the instrument.
Use only the proper Fuse
: Use the fuse type and rating specified for this product.
Use in proper Atmosphere : Please refer to operating conditions given in the
manual.
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Do not operate in wet / damp conditions.
•
Do not operate in an explosive atmosphere.
•
Keep the product dust free, clean and dry.
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Introduction
An Operational Amplifier, usually referred to as an 'Op-Amp' for brevity, Op-Amps
are among the most widely used electronic devices today, being utilized in a vast
array of consumer, industrial and scientific devices. In present days electronics system
a basic building block is the Operational Amplifier. The Operational Amplifier is a
versatile device that can be used to amplify DC input signal as well as AC input signal
and used for computing mathematical function such as addition, subtraction,
multiplication, integration and differentiation, and due to the ability to perform these
operations the name Operational amplifier stems.
With Scientech 2323, Op-Amp Applications student can study the basic applications
and will be able to perform the various application of operational amplifier. The OpAmps were used to model the basic mathematical operations addition, subtraction,
integration, differentiation, rectification, oscillation, filtering, peak detection,
comparision and so on. However, an ideal operational amplifier is an extremely
versatile circuit element, with a great many applications beyond mathematical
operations and to understand and performe those application it is nessesary to achive
beter understanding of its basic application. Thus Scientech 2323 has been divided
into different independent blocks for the ease of user to understand the various
application of operational amplifier. A function generator, generating Sine wave,
Square wave and triangular wave, and two variable DC supplies are provided on
board.
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Features
•
Self contained easy to operate
•
On board Function Generator.
•
On board test variable power supply.
•
Functional blocks indicated on board mimic.
•
Built in power supply.
•
Operating manual provided.
•
Compact size.
•
Ready experiments.
Technical Specifications
Function Generators
:
1.
Sine Wave
:
10Hz − 100 KHz (10VPP)
2.
Square Wave
:
10Hz − 100 KHz (10 VPP)
3.
Triangle Wave
:
10Hz − 100 KHz (8 VPP)
:
0-5V (variable)
:
0-5V (variable)
Power Supply
:
230V +/− 10%, 50 Hz
Power Consumption
:
4VA (Approx)
Test Points
:
28 nos
Dimensions (mm)
:
W 450 × H 113 × D 280
Weight
:
4Kg (Approx)
On board test Power Supplies
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Theory
The operational amplifier (Op-Amp) was designed to perform mathematical
operations. Although now superseded by the digital computer, Op-Amps are a
common feature of modern analog electronics. The Op-Amp is constructed from
several transistor stages, which commonly include a differential-input stage, an
intermediate-gain stage and a push-pull output stage. The differential amplifier
consists of a matched pair of bipolar transistors or FETs. The push-pull amplifier
transmits a large current to the load and hence has a small output impedance. At first
the Op-Amps are named as Ideal Op-Amp due to the salient parameters of the OpAmp are assumed to be perfect. There is no such thing as an ideal Op-Amp, but
present day Op-Amps come so close to ideal that Ideal Op-Amp analysis approaches
actual analysis. Op-Amps depart from the ideal in two ways. First, dc parameters such
as input offset voltage are large enough to cause departure from the ideal. The ideal
assumes that input offset voltage is zero. Second, ac parameters such as gain are a
function of frequency, so they go from large values at dc to small values at high
frequencies. This assumption simplifies the analysis, thus it clears the path for insight.
Although the ideal Op-Amp analysis makes use of perfect parameters, the analysis is
often valid because some Op-Amps approach perfection. In addition, when working at
low frequencies, several KHz, the ideal Op-Amp analysis produces accurate results,
but to understand Op-Amp several assumptions have to be made:
1.
First, assume that the current flow into the input leads of the Op-Amp is zero.
This assumption is almost true in FET Op-Amps where input currents can be
less than a pA, but this is not always true in bipolar high-speed Op-Amps where
tens of µA input currents are found.
2.
Second, the Op-Amp gain is assumed to be infinite, hence it drives the output
voltage to any value to satisfy the input conditions. This assumes that the OpAmp output voltage can achieve any value. In reality, saturation occurs when
the output voltage comes close to a power supply rail, but reality does not negate
the assumption, it only bounds it. Also, implicit in the infinite gain assumption
is the need for zero input signals. The gain drives the output voltage until the
voltage between the input leads (the error voltage) is zero.
3.
This leads to the third assumption that the voltage between the input leads is
zero. The implication of zero voltage between the inputs leads means that if one
input is tied to a hard voltage source such as ground, then the other input is at
the same potential. The current flow into the input leads is zero, so the input
impedance of the Op-Amp is infinite.
4.
Fourth, the output impedance of the ideal Op-Amp is zero. The ideal Op-Amp
can drive any load without an output impedance dropping voltage across it. The
output impedance of most Op-Amps is a fraction of an ohm for low current
flows, so this assumption is valid in most cases.
5.
Fifth, the frequency response of the ideal Op-Amp is flat; this means that the
gain does not vary as frequency increases. By constraining the use of the OpAmp to the low frequencies, we make the frequency response assumption true.
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Table1: Basic Ideal Op-Amp assumption
PARAMETER NAME
PARAMETERS SYMBOL
VALUE
IIN
0
Input offset voltage
VOS
0
Input impedance
ZIN
∞
ZOUT
0
a
∞
Input current
Output impedance
Gain
Ideal Op-Amp
Figure 1
But the Op-Amp is a linear amplifier with VOUT α VIN. The DC open-loop voltage
gain of a typical Op-Amp is 105 to106. The gain is so large that most often feedback is
used to obtain a specific transfer function and control the stability. The Op-Amp is
basically a differential amplifier having a large voltage gain, very high input
impedance and low output impedance. The Op-Amp has an "inverting" or negative
input and "noninverting" or positive input and a single output. The Op-Amp is usually
powered by a dual polarity power supply in the range of +/- 5 volts to +/- 15 volts.
The electrical parameters of a real Op-Amp are defined below:
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Maximum Output Voltage Swing:
The maximum output voltage, ±VOM, is defined as the maximum positive or negative
peak-output voltage that can be obtained without wave form clipping, when quiescent
DC output voltage is zero. ±VOM is limited by the output impedance of the amplifier,
the saturation voltage of the output transistors, and the power supply voltages. This is
shown in figure 2.
Maximum output voltage swing of Op-Amp.
Figure 2
The value of maximum output voltage which can be obtain without any clipping in
output voltage is always less then the power supply voltage.
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Large Signal Differential Voltage Amplification:
Large signal differential voltage amplification, AVD, is similar to the open loop gain of
the amplifier except open loop is usually measured without any load. This parameter
is usually measured with an output load. AVD is a design issue when precise gain is
required. The gain equation of a non inverting amplifier.
…….……….
(1)
β is a feedback factor, determined by the feedback resistors. The term in the equation
1/AVDβ is an error term. As long as AVD is large in comparison with 1/ β, it will not
greatly affect the gain of the circuit.
Input Capacitance:
Input capacitance, Ci, is measured between the input terminals with either input
grounded. Ci is usually a few pF.
Input Resistance:
Input resistance, ri is the resistance between the input terminals with either input
grounded. ri ranges from 107Ω to 1012Ω, depending on the type of input.
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Output Impedance:
Different data sheets list the output impedance under two different conditions. One is
closed-loop output impedance while another open-loop output impedance, both
designated by Zo. Zo is defined as the small signal impedance between the output
terminal and ground. Values of output impedance run from 50 Ω to 200 Ω. Common
emitter (bipolar) and common source (CMOS) output stages used in rail-to-rail output
Op-Amps have higher output impedance than emitter follower output stages. Output
impedance is a design issue when using rail-to-rail output Op-Amps to drive heavy
loads. If the load is mainly resistive, the output impedance will limit how close to the
rails the output can go. If the load is capacitive, the extra phase shift will erode phase
margin.
Effect of Output Impedance on Output Signal
Figure 3
Some new audio Op-Amps are designed to drive the load of a speaker or headphone
directly. They can be an economical method of obtaining very low output impedance.
Common-Mode Rejection Ratio:
Common-mode rejection ratio, CMRR, is defined as the ratio of the differential
voltage amplification to the common-mode voltage amplification,
CMRR = AD/ACM
…….………. (2)
Ideally this ratio would be infinite with common mode voltages being totally rejected.
The common-mode input voltage affects the bias point of the input differential pair.
Because of the inherent mismatches in the input circuitry, changing the bias point
changes the offset voltage, which, in turn, changes the output voltage.
The ADIF is the differential gain while the ACM is the common mode, and the value of
common mode gain is
ACM = VOCM/VCM
Where the VOCM is the output common mode voltage and VCM is the input common
mode voltage. Generally the is very small in comparison to; therefore, the CMRR
becomes a very large value. Being the very large value it is most often expressed into
decibel (dB). For IC 741 the CMRR is 90dB, and for IC 084 it is 120dB. CMRR falls
off as the frequency increases.
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Supply Voltage Rejection Ratio:
Supply voltage rejection ratio (SVRR), also known as, kSVR, power supply rejection
ratio, PSRR. SVRR is the ratio of power supply voltage change to output voltage
change. The power voltage affects the bias point of the input differential pair. Due to
the inherent mismatches in the input circuitry, changing the bias point changes the
offset voltage, which, in turn, changes the output voltage. For a dual supply Op-Amp,
…….………. (3)
This means the lower the value of SVRR, in micro volts better the performance of the
Op-Amp. The term ∆VCC± means that the plus and minus power supplies are
changed symmetrically. The SVRR is also represent as a DC parameter while When
kSVR is graphed vs. frequency, it falls off as the frequency increases.
Slew Rate at Unity Gain:
Slew rate, SR, is the rate of change in the output voltage caused by a step input. Its
units are V/µs or V/ms. Figure 4 shows slew rate graphically. The primary factor
controlling slew rate in most Op-Amps is an internal compensation capacitor CC,
which is added to make the Op-Amp unity gain stable. Referring to figure 5 voltage
change in the second stage is limited by the charging and discharging of the
compensation capacitor CC. The maximum rate of change is when either side of the
differential pair is conducting 2IE. Essentially SR = 2IE/CC. Remember, however,
that not all Op-Amps have compensation capacitors.
In Op-Amps without internal compensation capacitors, the slew rate is determined by
internal Op-Amp parasitic capacitances. Noncompensated Op-Amps have greater
bandwidth and slew rate, but the stability of the circuit is needed to take care by other
Means. In Op-Amps, power consumption is traded for noise and speed. In order to
increase slew rate, the bias currents within the Op-Amp are increased.
Slew Rate at Unity Gain as a function of time
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Figure 4
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Simplified Op-Amp Schematic
Figure 5
Total Harmonic Distortion plus Noise:
Total harmonic distortion plus noise, THD + N, compares the frequency content of
the output signal to the frequency content of the input. Ideally, if the input signal is a
pure sine wave, the output signal is a pure sine wave. Due to nonlinearity and noise
sources within the Op-Amp, the output is never pure. THD + N is the ratio of all other
frequency components to the fundamental and is usually specified as a percentage:
…….………. (4)
Figure 6 shows a hypothetical graph where THD + N = 1%. The fundamental is the
same frequency as the input signal. Nonlinear behavior of the Op-Amp results in
harmonics of the fundamental being produced in the output. The noise in the output is
mainly due to the input noise of the Op-Amp. All the harmonics and noise added
together make up 1% of the fundamental.
Two major reasons for distortion in an Op-Amp are the limit on output voltage swing
and slew rate. Typically an Op-Amp must be operated at or below its recommended
operating conditions to realize low THD.
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Output Spectrum with THD + N = 1%
Figure 6
Settling Time:
It takes a finite time for a signal to propagate through the internal circuitry of an OpAmp. Therefore, it takes a period of time for the output to react to a step change in the
input. In addition, the output normally overshoots the target value, experiences
damped oscillation, and settles to a final value. Settling time, ts, is the time required
for the output voltage to settle to within a specified percentage of the final value given
a step input. Figure 7 shows this graphically:
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Transient Response of Op-Amp
Figure 7
Settling time is a design issue in data acquisition circuits when signals are changing
rapidly. An example is when using an Op-Amp following a multiplexer to buffer the
input to an A to D converter. Step changes can occur at the input to the Op-Amp when
the multiplexer changes channels. The output of the Op-Amp must settle to within a
certain tolerance before the A to D converter samples the signal.
Unity Gain Bandwidth and Phase Margin:
There are five parameters relating to the frequency characteristics of the Op-Amp that
are:
1.
Unity-gain bandwidth (B1),
2.
Gain bandwidth product (GBW),
3.
Phase margin at unity gain (φm),
4.
Gain margin (Am), and
5.
Maximum output-swing bandwidth (BOM).
Unity-gain bandwidth (B1) and gain bandwidth product (GBW) are very similar. B1
specifies the frequency at which AVD of the Op-Amp is 1:
…….………. (5)
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GBW specifies the gain-bandwidth product of the Op-Amp in an open loop
configuration and the output loaded:
….………. (6)
GBW is constant for voltage-feedback amplifiers. It does not have much meaning for
current-feedback amplifiers because there is not a linear relationship between gain
and bandwidth.
Phase margin at unity gain (φm) is the difference between the amounts of phase shifts
a signal experiences through the Op-Amp at unity gain and 180°.
…….………. (7)
Gain margin is the difference between unity gain and the gain at 180_ phase shift
……… (8)
Maximum output-swing bandwidth (BOM) specifies the bandwidth over which the
output is above a specified value:
….………. (9)
The limiting factor for BOM is slew rate. As the frequency gets higher and higher the
output becomes slew rate limited and can not respond quickly enough to maintain the
specified output voltage swing. In order to make the Op-Amp stable, capacitor, CC, is
purposely fabricated on chip in the second stage (figure 5). This type of frequency
compensation is termed dominant pole compensation. The idea is to cause the openloop gain of the Op-Amp to roll off to unity before the output phase shifts by 180°.
Remember that figure 5 is very simplified, and there are other frequency shaping
elements within a real Op-Amp. Figure 8 shows a typical gain vs. frequency plot for
an internally compensated Op-Amp. As noted earlier, AVD falls off with frequency.
AVD (and thus B1 or GBW) is a design issue when precise gain is required of a
specific frequency band. Phase margin (φm) and gain margin (Am) are different ways
of specifying the stability of the circuit. Since rail-to-rail output Op-Amps have higher
output impedance, a significant phase shift is seen when driving capacitive loads. This
extra phase shift erodes the phase margin, and for this reason most CMOS Op-Amps
with rail-to-rail outputs have limited ability to drive capacitive loads.
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Voltage Amplification and Phase Shift vs. Frequency
Figure 8
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Experiment 1
Objective:
To study and observe Op-Amp as Voltage Comparator
Equipments Needed:
1.
Experiment board, Scientech 2323.
2.
Oscilloscope
3.
2 mm patch cords.
Comparator (Voltage Level Detector):
An Op-Amp comparator is a circuit which compares an arbitrary input signal against
a fixed reference voltage. The output of the comparator circuit switches between the
two saturation voltages depending on the value of arbitrary input signal with respect
to the reference voltage (if the input amplitude is less than the reference voltage,
output is at one saturation level and vice-versa). The purpose of the comparator is to
compare two voltages and produce a signal that indicates which voltage is greater.
The extremely large open-loop gain of an Op-Amp makes it an extremely sensitive
device for comparing its input with zero. The switching time for negative to positive
is limited by the slew rate of the Op-Amp.
The basic comparator will swing its output at VCC to VEE at the slightest difference
between its inputs. But there are many variations where the output is designed to
switch between two other voltage values. Also, the input may be tailored to make a
comparison to an input voltage other than zero. An Op-Amp can be used to compare 2
different voltages.
Noninverting Comparator: If you apply the input signal at the positive terminal of
the inputs and then use the negative input terminal to feed the reference voltage; the
output of the Op-Amp will go from high to low (or vice-versa) as the monitored
voltage crosses the reference voltage.
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Comparator (Voltage Level Detector)
Figure 9
As shown in figure 9 a non-inverting comparator circuit. A fixed reference voltage
Vref (say 1V or 2V…) is applied to the negative input (shown in figure 10 for Vref =
1V), and the other time varying signal voltage VIN is applied to the positive input of
Op-Amp. When VIN is less than Vref, the output voltage VOUT is at −VSAT (approx.
equal to −VEE) as the voltage at negative input terminal is higher than that of the
positive input terminal. On the other hand, when the positive input terminal voltage
VIN is greater than Vref, the positive input terminal becomes positive with respect to
the negative input and the VOUT bring switches to +VSAT (approx. equal to +VCC).
Here
VIN > Vref
then VOUT = +VSAT
VIN < Vref
then VOUT = − VSAT
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Input-Output Waveform of Noninverting Comparator
Figure 10
Thus, VOUT changes from one saturation level to another whenever VIN = Vref as
shown in figure 10. In short comparator is a type of analog-to-digital converter. At
any given time the VOUT shows whether VIN is greater or less than Vref. This is the
reason why it is also called a voltage level detector. In the similar way if the reference
voltage is negative with respect to ground. The above circuit is known as
Noninverting Comparator.
Inverting Comparator: The Voltage comparator may be noninverting or inverting
type. If the comparator output assumes the high state when the input voltage is above
a certain minimum level, then the comparator is assumed as inverting comparator.
That only happen when the input signal is feed into negative terminal of Op-Amp and
reference voltage is set at positive terminal in this case the output wave form look like
figure 11. The figure shows increment is duty cycle as the reference voltage increases,
It is due to the fact that now the DC reference is shifted to higher value, and
output signal polarity is opposite of input signal polarity.
VIN > Vref
then
VOUT = −VSAT
VIN < Vref
then
VOUT = +VSAT
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Input-Output Waveform of Inverting Comparator
Figure 11
Comparator needs protection from the damage due to excessive input voltage VIN.
Thus the diode D1 and D2 are used to protect the comparator from damage. Due to
diodes, the difference input voltage of Op-Amp (VIN − Vref), is clamped between 0.7V
to −0.7V. Hence these diodes are also called clamp diode, and are very necessary for
comparator protection.
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Comparator
Figure 12
Circuit diagram:
Figure 13
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Procedure:
•
For noninveritng comparator with positive reference
1.
Connect the patch cord as shown in figure 13.
2.
Connect the socket ‘IN1’ to 0-5 V DC supply as the reference voltage supply.
3.
Connect the on board function generator probe at socket ‘IN2’
4.
Set the 2V, 1 KHz input sinusoidal signal of function generator and observe the
input at oscilloscope CH II.
5.
Observe the output waveform between socket ‘1’ and ‘Gnd’, on oscilloscope
CH I.
6.
Note the amplitude, wave shape and duty cycle of the output waveform.
7.
Increase the reference voltage by the margin of 0.5V up to full range of DC
supply.
8.
Repeat the above steps from 4 to 6 for every increment in reference voltage.
9.
Connect the on board function generator probe at socket ‘IN1’ for inverting
comparator configuration
10.
Connect the socket ‘IN2’ to 0-5V DC power supply.
11.
Set the 2V, 1 KHz input sinusoidal signal of function generator and observe the
input at oscilloscope CH II.
12.
Observe the output waveform between socket ‘1’ and ‘Gnd’, on oscilloscope
CH I.
13.
Note the amplitude, wave shape and duty cycle of the output waveform.
14.
Increase the reference voltage by the margin of 0.5V up to full range of DC
supply.
15.
Repeat the above steps from 11 to 13 for every increment in reference voltage.
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Observation Table:
S. No.
VIN
Input voltage
(volt)
Vref
Reference voltage
(volt)
VOUT
Output voltage
(volt)
Duty
Cycle
(Measured )
Observation diagram:
1.
For Non inverting Comparator with positive reference Voltage:
2.
For Inverting Comparator with positive reference voltage:
Conclusion: The duty cycle of Comparator change as the reference voltage changes.
I.e. duty cycle decreases with the increase in positive reference voltage
in Noninverting comparator; whilst there is an increment in duty cycle
for the same condition in Inverting amplifier.
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Experiment 2
Objective:
To study and observe Op-Amp as Zero Crossing Detector
Equipments Needed:
1.
Experiment board, Scientech 2323.
2.
Oscilloscope,
3.
2mm patch cords.
Zero Crossing Detector (Sine wave-to-Square Wave Converter):
As the name indicates the zero crossing detector is a device for detecting the point
where the voltage crosses zero in either direction.
What happens to an operational amplifier if the negative feedback is removed? With
no feedback and very high gain, obviously the output voltage will go to one extreme
limit or the other. Typically this is limited to just outside the ±10 volt limit used in
analog computers, and is inherently current-limited to avoid any possible damage. But
is there really any use for such a circuit? This circuit operates as a zero crossing
detector.
Basic Inverting Zero crossing detector
Figure 14
The circuit in figure 14, its output changes polarity whenever the input voltage
crosses zero to change polarity. In the configuration shown, the output voltage
polarity is opposite to the input polarity. This configuration is known as Inverting
Zero crossing detector. It produces a True or logic 1 output whenever the input
voltage goes negative. As such, it can also operate as a sign detector. An immediate
application of the zero crossing detectors is the Sine to square converter. However,
the two inputs can be swapped, in which case VOUT will have the same polarity as
VIN. This is known as Noninverting amplifier. It produces a True or logic 1 output
whenever the input goes positive and a False or logic 0 when input goes negative.
Zero Crossing Detector is an immediate application of the Comparator, only in this
circuit the reference voltage is set to zero. Figure 15 shows the output wave from of a
zero crossing detector works. The below shown circuit can also be used as a Sine to
Square wave generator provided that Vref is set to zero (Vref = 0).
Here
If VIN = Positive cycle;
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Then VOUT = −VSAT
If VIN = negative cycle;
Then VOUT = +VSAT
Zero-crossing Detector wave form
Figure 15
However, this circuit is still limited because it cannot detect any other input voltage
than zero. In a wide range of situations, we would like to be able to detect whether or
not the input is above (or below) some arbitrarily specified non-zero voltage.
Zero crossing detectors needs protection from the damage due to excessive input
voltage VIN. Thus the diode D1 and D2 are used to protect the comparator from
damage. Due to diodes, the difference input voltage of Op-Amp (VIN − Vref), is
clamped between 0.7V to −0.7V. Hence these diodes are also called clamp diode, and
are very necessary for comparator protection.
Zero Crossing Detector
Figure 16
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Circuit diagram:
Figure 17
Procedure:
•
Connect the socket ‘IN1’ to ground ‘Gnd’ opposite of figure 17.
1.
Connect the on board function generator probe at socket ‘IN2’
2.
Set the 1 V, 1 KHz input sinusoidal signal of function generator and observe the
input at oscilloscope CH II.
3.
Observe the output waveform between socket ‘1’ and ‘Gnd’, on oscilloscope
CH I.
4.
Note the amplitude of the output waveform.
5.
Increase the voltage by the margin of 1V up to full range of function generator.
6.
Repeat the above steps from 2 to 4.
7.
Connect the on board function generator probe at socket ‘IN1’ for inverting zero
crossing detector.
8.
Connect the patch cord as shown in figure 17.
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9.
Connect the socket ‘IN2’ to ground ‘Gnd’.
10.
Set the 1 V, 1 KHz input sinusoidal signal of function generator and observe the
input at oscilloscope CH II.
11.
Observe the output waveform between socket ‘1’ and ‘Gnd’, on oscilloscope
CH I.
12.
Note the amplitude of the output waveform.
13.
Repeat the above steps 10 to 12.
Observation diagram:
1.
For Noninverting Zero Crossing Detector:
2.
For Inverting Zero Crossing Detector:
Conclusion:
1.
A Noninverting zero crossing detector gives a square wave and the phase
difference between input and output signal is 180°.
2.
Inverting Zero crossing detector gives a square wave with the 0° or 360° phase
difference between input and out put signal and the output waveform, is same as
it is shown in figure 15.
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Experiment 3
Objective:
To observe the Op-Amp working as Logarithmic Amplifier
Equipments Needed:
1.
Experiment board, Scientech 2323
2.
Oscilloscope
3.
2 mm patch cords.
Logarithmic Amplifier:
A log amplifier simply gives the logarithmic output of signal at its input i.e. reducing
a signal logarithmically. Using simple circuitry and a high performance Op-Amp it is
possible to produce logarithmic and anti-logarithmic or exponential amplifiers having
good linearity. Such amplifiers use the nonlinear volt-ampere relationship of the p-n
junction itself of a forward active biased bipolar Transistor, this relationship is given
by
Ic = Is [exp (Vbe/VT)-1]
…….………. (10)
Where Ic is collector current,
Is is the reverse saturation current, closer to pA range,
Vbe is the base emitter voltage drop,
VT is the thermal voltage kT/q=26 mV at room temperature,
In practice Ic >> Is hence Eq.10 can be approximated as:
Ic = Is [exp (Vbe/VT)]
…….………. (11)
This represents the perfect exponential law. Dividing both sides by is and taking their
logarithm yields
Vbe = VT ln (Ic/Is)
…….………. (12)
Eq.12 represents the perfect logarithmic law.
Basic Log Amplifier
Figure 18
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Consider the above basic transdiode configuration of log amplifier in which the
bipolar transistor is included in the feedback path on an Op-Amp. Referring to the
current Is must be equal to the collector current. Thus the circuit forces the collector
current of the device to be proportional to the input voltage Since Vbe is
logarithmically related to the collector current , and since the output voltage Vo is just
the base-emitter voltage of the deice, we have
I1= Is exp (Vbe/VT)
…….………. (13)
As given by Eq.11 and Vo = -Vbe. Thus
Vo = -VT ln (I1/Is)
In case of above figure the resistor R1 serves to convert the input voltage into a current
(Vs/R1) because of virtual ground at inverting. Thus,
Vo = -VT ln (Vs/IsR1)
…….………. (14)
This relation shows that Vo is proportional t o the natural logarithm of Vs. The term
IsR1 acts as a scale factor, that is, ln (AVs) is generated, where the scale factor A is set
by R1. Additional gain can be obtained by connecting Vo to a linear amplifier. We
note that for this log- amplifier to operate properly Vs must be positive. Thus this is a
unipolar device.
Important Points:
1.
Diode is connected between the outputs and inverting pin of the Op-Amp to
protect the Base Emitter junction from excessive reverse voltage (The anode of
the diode should be connected to the output pin.)
2.
The Transdiode (Transistor) circuit has a tendency to oscillate due the presence
of an active element in the feedback path that can provide gain rather than loss.
To overcome the instability the circuit requires frequency compensation.
3.
The feedback capacitor C is connected to combat the stray capacitance, and the
inverting input.
4.
Input voltage limitation is due to the dynamic range of the Antilog amplifier
circuit component’
5.
Antilog or exponential amplifier circuit output is exponential of the input hence
for practical aspect input and output are measured as peak to peak voltage; it
overcomes the positive and negative input cycle problem.
6.
This circuit are very basic hence some times it may give a clipped output instead
of full sine wave.
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Applications:
A Logarithmic amplifier has property to compress the signal and Antilog or
Exponential amplifier to expand the input signal. Hence these two circuits have their
basic application in Companding i.e. a technique to compress the high amplitude
signal at transmitter and expand the same at the receiver for noise reduction.
Circuit diagram:
. Figure 19
Procedure:
•
Connect the patch cord as shown in figure 19.
1.
Connect the on board function generator probe at socket ‘IN3’
2.
Set the 3V, 1 KHz input sinusoidal signal of function generator and observe the
input at oscilloscope CH II.
3.
Observe the output waveform between sockets ‘2’ and ground, on oscilloscope
CH I.
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Observation of Logarithmic Waveform:
Conclusion: Output waveform shows the logarithmic conversion of input sinewave.
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Experiment 4
Objective:
To observe the Op-Amp working as antilogarithmic Amplifier
Equipments Needed:
1.
Experiment board, Scientech 2323.
2.
Oscilloscope
3.
2mm patch cords.
Antilog Amplifier:
An Antilog amplifier gives the exponential output of signal at its input i.e. amplifying
a signal exponentially. The basic Log Amplifier can be rearranged to form Antilog
Amplifier as shown in figure 20. The negative part of sinusoidal input forward bias
the BE junction of Transistor. A diode across the input (between emitter and ground
with anode connected to emitter) may be connected to protect the BE junction from a
possible excessive reverse voltage.
Basic Antilog Amplifier
Figure 20
Consider the above basic transdiode configuration of antilog amplifier in which the
bipolar transistor is included in the input path of an Op-Amp.
For the transistor we have the relation given by:
Ic = Is exp (Vbe/VT)
Since Vbe = -Vs as seen in the figure Ic can be expressed as
Ic = Is exp (-Vs/VT)
Since Ic must flow from the Op-Amp output through Rf, The Op-Amp output is
Vo = Rf Is exp (|Vs|/VT)
The basic antilog amplifier suffers from the same drawbacks those discussed for basic
log amplifier.
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Important Points:
1.
The Trandiode (Transistor) circuit has a tendency to oscillate due the presence of
an active element in the feedback path that can provide gain rather than loss. To
overcome the instability the circuit requires frequency compensation.
2.
The feedback capacitor C is connected to combat the stray capacitance, and the
inverting input.
3.
Input voltage limitation is due to the dynamic range of the Antilog amplifier
circuit component’
4.
Antilog or exponential amplifier circuit output is exponential of the input hence
for practical aspect input and output are measured as peak to peak voltage; it
overcomes the positive and negative input cycle problem.
5.
This circuit are very basic hence some times it may give a clipped output instead
of full sine wave.
Applications:
A Logarithmic amplifier has property to compress the signal and Antilog or
Exponential amplifier to expand the input signal. Hence these two circuits have their
basic application in Companding i.e. a technique to compress the high amplitude
signal at transmitter and expand the same at the receiver for noise reduction.
Circuit diagram:
Figure 21
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Procedure:
•
Connect the patch cord as shown in figure 21.
1.
Make Connections according to the procedure of experiment 3.
2.
Connect Socket 2 of logarithmic amplifier to IN4 of anti log amplifier.
3.
Observe the output waveform between sockets ‘3’ and ground, on oscilloscope
CH I.
Observation of Anti Log Waveform:
Conclusion: On Providing output of Logarithmic Amplifier to the input of Antilog
Amplifier we can recover the sinusoidal waveform, as at the input of
Logarithmic Amplifier.
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Experiment 5
Objective:
To study and observe Op-Amp as a Peak Detector
Equipments Needed:
1.
Experiment board, Scientech 2323.
2.
Oscilloscope
3.
2 mm patch cords.
Peak Detector:
A Peak Detector detects the peak value of the input, i.e., VO= + VIN (peak). It is also
known as Envelope Detector or Diode Detector. Why do we need a peak detector
when we have an AC voltmeter to detect the amplitude of AC signal? An AC
voltmeter cannot be used to measure the non-sinusoidal waveforms, such as square,
triangular, etc. because it is designed to measure the rms value of pure sine wave.
Hence the possible approach is to measure peak values of non-sinusoidal waveforms.
To detect the peak value, the circuit follows the input signal until the peak value is
reached. This value then held indefinitely until a new, larger peak comes along. In
which case the circuit would update its output to the new peak value. There are two
types of peak detector, positive peak detector and negative peak detector.
Positive peak detector: It detects the positive peak value of non-sinusoidal signal and
provides the positive peak value at (DC voltage) at the output. figure 22 shows a peak
detector that measures the positive peak values of the Sin wave/square wave input;
VIN.
Positive peak detector
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Figure 22
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But how does a Positive peak detector work? The operation of peak detector is:
During the positive half cycle of VIN the output of the Op-Amp makes D1 ‘ON’,
which charges the capacitor C to the positive peak value of the input voltage, VIN.
Thus, when D1 is forward biased, the amplifier operates as a voltage follower. During
negative half of input square wave, D1 is reversed biased, and voltage across C is
retained. The only discharge path for C is through load RL, since the input bias current
IB is negligible .for proper operation following equations should be satisfied
CRd < T / 10
CRL > 10 T
………....…. (15)
Here Rd, is the resistance of the forward biased diode, 100 typically,
And T is the time period of the input waveform. If RL is very small so that the eq.15
cannot be satisfied, we can use a buffer, i.e., voltage follower between capacitor and
load resistor RL. Figure 23 shows the output waveform.
Output of Positive Peak Detector
VO = + VIN (peak)
Output waveform of positive peak detector
….………. (16)
Figure 23
In the circuit of figure 22, the resistance R is used to protect the Op-Amp against
excessive discharge currents, especially when the power supply is Switched-off. The
resistor RM = R minimizes the offset due to input currents. The diode D2 conducts
during the negative half – cycle of VIN and hence prevents the Op-Amp from going
into negative saturation. This in turn helps to reduce the recovery time of the OpAmp.
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Negative peak detector: It detects the negative peak value of non-sinusoidal signal
and provides the negative peak value at (DC voltage) at the output. Figure 22 shows a
peak detector that measures the positive peak values of the Sin wave/square wave
input; VIN. To detect the negative peaks of a signal, we have to simply reverse the
polarity of diodes D1 and D2. During the negative half cycle of VIN the output of the
Op-Amp makes D1 ‘ON’, which charges the capacitor C to the negative peak value of
the input voltage, VIN.
Thus, when D1 is forward biased (in negative cycle), the amplifier operates as a
voltage follower. During positive half of input signal, D1 is reversed biased, and
voltage across C is retained and discharge path for C is through load RL.
Output of Negative Peak Detector
VO = - VIN (peak)
…….………. (17)
Circuit diagram:
Figure 24
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Figure 25
Procedure:
•
Connect the patch cord as shown in figure 24, for positive peak detector.
1.
Connect the on board function generator probe at socket ‘IN5’
2.
Connect the socket ‘a1’ to ‘a2’ and connect socket ‘a4’ to ‘a5’.
3.
Set the input at 1V, 1 KHz sinusoidal/square signal of function generator and
observe the input at oscilloscope CH II.
4.
Observe the output waveform between sockets ‘4’ and ground, on oscilloscope
CH I, with DC coupling.
5.
Note the amplitude of output waveform and ripple in it.
6.
Verify the measured output with the calculated output using eq.16.
7.
Increase the input amplitude up to 10 V with the margin of 1 V.
8.
Repeat the above steps 4 to 6 for every increment in input voltage.
9.
Connect the socket ‘a1’ to ‘a3’ and connect socket ‘a4’ to ‘a6’ for the negative
peak detector configuration.
10.
Connect the on board function generator probe at socket ‘IN5’
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11.
Set the input at 1V, 1 KHz sinusoidal/square signal of function generator and
observe the input at oscilloscope CH II.
12.
Observe the output waveform between sockets ‘4’ and ground, on oscilloscope
CH I, with DC coupling.
13.
Note the amplitude of output waveform and ripple in it.
14.
Verify the output with the calculated output using eq.17.
15.
Increase the input amplitude up to 10 V with the margin of 1 V.
16.
Repeat the above steps 11 to 13 for every increment in input voltage
Observation Table:
S. No.
Conclusion:
Input Voltage
VIN (VPeak)
Output Voltage
VOUT
Peak detector capture the positive peak in positive peak detector
configuration i.e. give the positive DC voltage. The negative peak in
negative peak detector configuration i.e. give the negative DC
voltage.
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Experiment 6
Objective:
To study and observe Op-Amp as a Wien Bridge Oscillator and its gain factor for a
smooth sine wave.
Equipments Needed:
1.
Experiment board, Scientech 2323.
2.
Oscilloscope
3.
Multi-meter,
4.
Frequency counter
5.
2 mm patch cords.
Oscillator:
Oscillators are circuits that produce periodic waveforms without input other than
perhaps a trigger. They generally use some form of active device, lamp, or crystal,
surrounded by passive devices such as resistors, capacitors, and inductors, to generate
the output.
There are two main classes of oscillator: relaxation and sinusoidal. Relaxation
oscillators generate the triangular, saw tooth and other non sinusoidal waveforms.
Sinusoidal oscillators consist of amplifiers with external components used to generate
oscillation, or crystals that internally generate the oscillation. The focus here is on
sine wave oscillators, created using operational amplifiers Op-Amps. Sine wave
oscillators are used as references or test waveforms by many circuits.
An oscillator is a type of feedback amplifier in which part of the output is fed back to
the input via a feedback circuit. If the signal fed back is of proper magnitude and
phase, the circuit produces alternating currents or voltages. To find the requirement of
oscillator consider the block diagram in figure 26, this block diagram looks identical
of the feed back amplifier. However the input voltage VIN is zero. Also the feedback
is positive because most oscillators use positive feedback. Finally, the closed-loop
gain of the amplifier is denoted as AV rather then AF.
Block diagram of Oscillator
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Using the above equation the relationship obtain is
…….………. (18)
Two requirements for oscillation are:
1.
The magnitude of the loop gain AVB must be at least 1.
2.
The total phase shift of the loop gain AvB must be equal to 0° or 360°. If the
amplifier causes a phase shift of 180°, the feedback circuit must provide an
additional phase shift of 180° so that the total phase shift around the loop is
360°.
The type of wave form generated by an oscillator is depends of the components used
in circuits and hence the waveform generated can be any thing from Sine, Square, or
triangular. The frequency of the oscillation is also determined by the component in
feedback circuit.
But still the question arises; what is the need of oscillator? And where do we use
them? Lets go to the start again .First, what is the oscillator? It is a device that works
based on oscillation. Well, what is that? It is the movement of two things that work on
the energy flow they receive. An oscillating fan, clock and transmitters work by
working on the energy. In the example of a clock, pendulum, the oscillator keeps time
for us accurately based on the principals of oscillation. This is a simple type of
oscillator.
Do you still think you have not use an oscillator in your lifetime? If so, think again.
They are in most computers, clocks of all sorts, as well as in watches, metal detectors,
radios of all powers and uses, as well as many mechanical devices. The oscillator is
one of the most important instruments in our life because it helps us to tell time
accurately. The work of oscillator is not stopped here, but they are used in a variety of
ways throughout our lives. For example, you will find them located not only in clocks
but also in electronic devices of all types. For example, audio frequency equipment
has them as well as wireless receivers and transmitters as well. You will find them in
a sensitive amplifier or you will find them in signals that are used and sent out. Their
uses are many and far between.
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Wien Bridge Oscillator:
The Wien Bridge is one of the simplest and best known oscillators and is used
extensively in circuits for audio applications. Figure 27 shows the basic Wien Bridge
circuit configuration. On the positive side, this circuit has only a few components and
good frequency stability.
Because of its simplicity and stability, it is the most commonly used audio-frequency
oscillator. In the Wien Bridge circuit is connected between the amplifier input
terminals and the output terminal. The bridge has a series RC network in one arm and
a parallel RC network in the adjoining arm. In the remaining two arms of the bridge,
resistor R1 and Rf are connected.
The phase angle criterion for oscillation is that the total phase shift around the circuit
must be 0°. This condition occurs only when the bridge is balanced, that is at
resonance. The frequency of oscillation FO is exactly the resonant frequency of the
balanced Wien Bridge and is given by
Here FO is the frequency generated by Wien bridge oscillator,
FO
= 1/2 π RC
= 0.159 / RC
…………… (19)
Assuming that the resistors are equal in the value, and the capacitors are equal in the
value in the reactive leg of the Wien Bridge. At this frequency the gain required for
sustained oscillation is given by
Av = 1/B = 3
That is,
…………… (20)
1+ Rf / R1 = 3
Or
Rf = 2R1
…………… (21)
Wien-Bridge Circuit Schematic
Figure 27
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Circuit diagram:
Figure 28
Procedure:
•
To generate the sine wave by Wien bridge Oscillator.
1.
Calculate the value of R to generate the 8 KHz frequency by the eq.19.
2.
Connect the probes of multi-meter at tp7 and tp8 and rotate the dual
Potentiometer P2 till the value of Potentiometer is equal to the calculated R,
3.
Connect the socket ‘b1’ to socket ‘b2’ to complete the bridge.
4.
Connect the oscilloscope probe at tp6 and ground ‘Gnd’.
5.
If the signal is little bit distorted vary the Potentiometer P1 a little till the perfect
sine wave come.
6.
Note the output amplitude by oscilloscope and frequency by using frequency
counter, match it with measured frequency.
7.
Disconnect the patch cord between ‘b1’ and socket ‘b2’, and measure the value
of Potentiometer P2, between tp5 and socket ‘b1’.
8.
The value of P1 should be twice of the resistance R7.
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9.
Verify the P1 value by using eq.21.
10.
Calculate the gain of oscillator and verify it by using eq.20.
11.
Calculate the value of R for the frequency up to 10 KHz with the margin of 1
KHz.
12.
Repeat the above steps form 2 to 10.
Observation Table:
S.
No.
Frequency
(f)
output
R
voltage
(Calculated ) V
OUT
output
frequency
fOUT
(measured)
RF
Feed
back
resistance
Gain
Conclusion:
1.
The output is a perfect sine wave and the frequency varies with the variation in
the combination of RC.
2.
Value of RF; Feed back resistance is twice of the R1.hence the gain of the bridge
is 3.
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Experiment 7
Objective:
To study and observe Op-Amp as a Phase Shift Oscillator and its phase shift at every
RC combination.
Equipments Needed:
1.
Experiment board, Scientech 2323.
2.
Multi-meter,
3.
Oscilloscope,
4.
2mm patch cords.
Phase Shift Oscillator:
The phase shift oscillator produces positive feedback by using an inverting amplifier
and adding another 180° of phase shift with the three high-pass filter circuits. It
produces this 180° phase shift for only one frequency.
First question that comes into out mind is how dose this signal generate? The
operation of the RC Phase Shift Oscillator can be explained as follows. The starting
voltage is provided by noise, which is produced due to random motion of electrons in
resistors used in the circuit. The noise voltage contains almost all the sinusoidal
frequencies. This low amplitude noise voltage gets amplified and appears at the
output terminals. The amplified noise drives the feedback network which is the phase
shift network. Because of this the feedback voltage is maximum at a particular
frequency, which in turn represents the frequency of oscillation. Furthermore, the
phase shift required for positive feedback is correct at this frequency only. The
voltage gain of the amplifier with positive feedback is given by from the above
equation we can see that if
. The gain becomes infinity means that
there is output without any input. i.e. the amplifier becomes an oscillator. This
condition
is known as the Barkhausen criterion of oscillation. Thus the output
contains only a single sinusoidal frequency. In the beginning, as the oscillator is
switched on, the loop gain Aβ is greater than unity. The oscillations build up. Once a
suitable level is reached the gain of the amplifier decreases, and the value of the loop
gain decreases to unity. So the constant level oscillations are maintained. Satisfying
the above conditions of oscillation the value of R and C for the phase shift network is
selected such that each RC combination produces a phase shift of 60°. Thus the total
phase shift produced by the three RC networks is 180°. Therefore at the specific
frequency fo the total phase shift from the base of the transistor around the circuit and
back to the base is 360° thereby satisfying Barkhausen criterion. The mathematics for
calculating the oscillation frequency and oscillation criteria for this circuit are
surprisingly complex, due to each R-C stage loading the previous ones. The
calculations are greatly simplified by setting all the resistors (except the negative
feedback resistor) and all the capacitors to the same values. In the diagram, if R1 = R2
= R3 = R, and C1 = C2 = C3 = C, then:
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…….………. (22)
This frequency, the feedback factor of the network is
. In order that
it
is required that the amplifier gain
for oscillator operation. Figure 29 shows a
phase shift oscillator, which consists of an Op-Amp as the amplifying stage and three
RC cascaded networks as the feedback voltage from the output back to the input of
the amplifier. The Op-Amp is used in the inverting mode; therefore, any signal that
appears at the inverting terminal is shifted by 180° at the output. An additional 180°
phase shift required for oscillation is provided by the cascaded RC networks. Thus the
total phase shift around the loop is 360° (or 0°). The most common way of achieving
this kind of filter is using 3 cascaded resistor-capacitor filters, which produce no
phase shift at one end of the frequency scale, and a phase shift of 270° at the other end
Figure 29
And for
Hence
A = 29
......................... (23)
Rfeedback = 29R
..........................(24)
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One of the simplest implementations for phase shift oscillator uses an operational
amplifier (Op-Amp), 3 capacitors and 4 resistors, as shown in the figure 30.
Figure 30
A phase-shift oscillator can be built with one Op-Amp is shown above the normal
assumption is that the phase shift sections are independent of each other. Then
Equation is written
AB = A [l / RCs + 1]3
The loop phase is -180° when the phase shift of each section is -60°, and this occurs
when ω = 1.732 / 2πRC because the tangent of 60° = 1.732. The oscillation frequency
with the component values shown in figure 30 is slightly different than the calculated
oscillation frequency. These discrepancies are partially due to the component
variations, but the biggest contributing factor is the incorrect assumption that the RC
section does not load each other. This circuit configuration was very popular when
active components were large and expensive, but now Op-Amps are inexpensive and
small and come four in a package, so the single Op-Amp phase-shift oscillator is
losing popularity.
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Circuit diagram:
Figure 31
Procedure:
•
To observe the working of Phase Shift Oscillator.
1.
Calculate the value of R to generate the 1 KHz frequency by the eq.22.
2.
Connect the probes of multi-meter at tp9 and ground ‘Gnd’ and rotate the dual
Potentiometer P3 till the value of Potentiometer is equal to the calculated R,
3.
Connect the socket ‘e1’ to socket ‘e2’ to complete the bridge.
4.
Connect the oscilloscope CH I probe at tp12 and ground ‘Gnd’ to observe the
output VOUT.
5.
Vary the Potentiometer P4 till the perfect wave occurs.
6.
Connect the Oscilloscope CH II probe at tp10 to observe the phase shift between
output and 1st RC combination output, V1OUT.
7.
Disconnect the Oscilloscope CH II probe and connect it tp9 to observe the phase
shift between output and 2nd RC combination V2OUT.
8.
Check the phase difference between tp9 and tp10, by connecting them with
Oscilloscope CH I and CH II respectively.
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9.
Disconnect the patch cord between ‘e1 ’ and socket ‘e2’, and measure the value
of Potentiometer P4, between tp11 and socket ‘e1’.
10.
The value of P1 should be twenty nine times of the resistance R8.
11.
Calculate the gain by using eq.23.
12.
Calculate the value of R for the frequency up to 10 KHz with the margin of
1 KHz.
13.
Repeat the above steps form 2 to 12.
Note: To measure the phase shift take only one pair of node and antinode of the
signal, count the blocks it take, (you can also decrease the frequency for more
blocks), divide the 360 by number of blocks, That will give you the estimation of how
much degree are there in one block. It is to make your calculation easy.
Observation Table:
S. No.
frequency
(f)
R
Phase
shift
Phase
shift
Phase
shift
(Calculated)
Φ
Φ
Φ
(VOUT V1OUT)
(VOUT –
V2OUT)
(V1OUT –
V2OUT)
Conclusion:
1.
The phase shift between is VOUT - V1OUT = 60°.
2.
The phase shift between is VOUT – V2OUT = 120°
3.
The phase shift between is V1OUT – V2OUT = 60°.
4.
The value of the feedback resistance is 29 times higher then that of R8, thus the
value of gain is 29.
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Experiment 8
Objective:
To study and observe Op-Amp as a Function generator, generating Square and
Triangle wave.
Equipments Needed:
1.
Experiment board, Scientech 2323.
2.
Multi-meter,
3.
2 mm patch cords,
4.
Oscilloscope.
Square wave generator: A square wave is a basic kind of non-sinusoidal waveform
encountered in electronics and signal processing. An ideal square wave alternates
regularly and instantaneously between two levels, which may or may not include zero.
The circuit at figure 32 uses a comparator with both positive and negative feedback to
control its output voltage. Because the negative feedback path uses a capacitor while
the positive feedback path does not, however, there is a time delay before the
comparator is triggered to change state. As a result, the circuit oscillates, or keeps
changing state back and forth at a predictable rate. Because no effort is made to limit
the output voltage, it will switch from one extreme to the other.
Square wave generator
Figure 32
If we assume it starts at -10 volts, then the voltage at the "+" input will be set by R2
and R1 to a fixed voltage equal to-10R1/ (R1 + R2) volts. This then becomes the
reference voltage for the comparator, and the output will remain unchanged until the
"-" input becomes more negative than this value. But the "-" input is connected to a
capacitor (C) which is gradually charging in a negative direction through resistor Rf.
Since C is charging towards -10 volts, but the reference voltage at the "+" input is
necessarily smaller than the -10 volt limit, eventually the capacitor will charge to a
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voltage that exceeds the reference voltage. When that happens, the circuit will
immediately change state. The output will become +10 volts and the reference voltage
will abruptly become positive rather than negative. Now the capacitor will charge
towards +10 volts, and the other half of the cycle will take place.
The output frequency is given by the approximate equation:
FOUT
= 1/ {2RfC ln (1+ 2R1/R2)}
In practice, circuit values are chosen such that R1 is approximately Rf/3, and R2
is in the range of 2 to 10 times R1.
Square waves are universally encountered in digital switching circuits and are
naturally generated by binary (two-level) logic devices. They are used as timing
references or "clock signals", because their fast transitions are suitable for
triggering synchronous logic circuits at precisely determined intervals. However,
as the frequency-domain graph shows, square waves contain a wide range of
harmonics; these can generate electromagnetic radiation or pulses of current that
interfere with other nearby circuits, causing noise or errors. To avoid this
problem in very sensitive circuits such as precision analog-to-digital converters,
sine waves are used instead of square waves as timing references. In musical
terms, they are often described as sounding hollow, and are therefore used as the
basis for wind instrument sounds created using subtractive synthesis.
Additionally, the distortion effect used on electric guitar clips the outermost
regions of the waveform, causing it to increasingly resemble a square wave as
more distortion is applied.
Triangular wave generator:
An Oscillator which generator a Triangular wave is known as Triangular wave
generator. A triangle wave is a basic kind of non-sinusoidal waveform named for its
triangular shape. Like a square wave, the triangle wave contains only odd harmonics.
However, the higher harmonics roll off much faster than in a square wave
(proportional to the inverse square of the harmonic number as opposed to just the
inverse), and so its sound is smoother than a square wave and is nearer to that of a
sine wave. How to generate a triangle wave? An Op-Amp integrator can be use to
obtain a linear triangle wave along with the square wave? A separate integrator is
being used to generate a ramp voltage from the generated square wave. As a result,
we can get both waveforms from a single circuit. The phase relationship shown
between the two output waveforms is that the integrator inverts as well as integrating,
so it will produce a negative-going ramp for a positive input voltage, and vice-versa.
The simple Tri-wave generator has become an often used analog circuit. Tri-wave
oscillators are more easily designed, require less circuitry, and are more easily
stabilized than sine wave oscillators. Further, the highly linear output of Tri-wave
generators make them useful in many ``sweep'' circuits and test equipment. Figure 33
shows a simple square & triangular wave generator.
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Square & Triangular wave generator
Figure 33
Because we are now using an Op-Amp integrator to get the triangle wave, the
equation for the operating frequency is simplified
FOUT = (1/4RtC) {R2/ R1}
…………. (25)
The square wave amplitude is still the limit of voltage transition, which we are
assuming here to be ±10 volts. The triangle wave's amplitude is set by the ratio of
R1/R2. The frequency can be changed by changing the value of Rt.
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Circuit diagram:
Figure 34
Procedure:
•
To observe the operation amplifier in function generator mode generating square
and triangle wave.
1.
Rotate the Frequency adj. Potentiometer P5 (for coarse frequency) to initial
position and do same with Potentiometer P6 (for fine frequency).
2.
Connect the Oscilloscope CH1 at tp13 and ground.
3.
Observe and note the amplitude, wave shape and frequency of output signal.
4.
Vary the amplitude Potentiometer P7, to max and see the maximum output.
5.
Rotate the frequency Potentiometer and note the variation in the frequency.
6.
Disconnect the Oscilloscope probe and connect it to tp14. And ground.
7.
Rotate the Frequency adj. Potentiometer P5 (for coarse frequency) to initial
position and do same with Potentiometer P6 (for fine frequency).
8.
Repeat the above steps from 3 to 5.
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Observation Table:
S. No.
Output Signal
Output Voltage
VOUT
Output Frequency
Range
S. No.
Output Signal
Output Voltage
VOUT
Output Frequency
Range
Conclusion:
1.
The maximum amplitude of the signal is equal to Voltage swing.
2.
Frequency ranges is in between 100Hz to 1 KHz.
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Experiment 9
Objective:
To study and observe Op-Amp as a Half Wave Precision Rectifier
Equipments Needed:
1.
Experiment board, Scientech 2323.
2.
2 mm patch cords.
3.
Oscilloscope
Precision Rectifier:
Rectification is a process whereby Alternating Current (AC) is converted into Direct
Current (DC), or Rectification is a process of separating the positive and negative
portion of waveform from each other and selecting from them what part of the signal
to retain. In the case of half-wave rectification, we can choose to keep one polarity
(say, positive or negative) while discarding the other. A full-wave rectifier keeps both
halves of the input signal, and yet renders them both with the same output polarity.
Rectifier is an electrical device, comprising one or more semiconductor devices. Half
wave rectification can be achieved with a single diode. An ideal diode will work as a
switch, which will behave as a short for forward bias and open for riverse bias signal
i.e. it will pass every positive signal applied on its anode and block every signal of
negative polarity applied on its anode, and the output will be a half wave signal;a
pulsuating DC signal that increas to maximum then decrease to zero and remain zero
for rivers bias of diode, and the peak input voltage will be equal to peak output
voltage,
VP(OUT) = VP(IN)
But a practical diode has its won limitation and it will nerver give a perfect half wave
due to its barrier potentioal which is also known as knee voltage; a knee voltage is the
minimum voltage applied on the diode after which a diode start conducting
(approximately 0.7 V). Because of this potential barrier the diode doesn’t turn on until
the AC sourece voltage reaches approximately 0.7 V.
Thus,
VP(OUT) = VP(IN) - 0.7 V
……………… (26)
If the inuput amplitude is very high, say, hundreds of volt then output amplitude will
be very close to perfect half wave voltage. But if the output amplitude is in range of
tenth of voltage the output will not be a perfect half wave it will be 0.7V minus the
peak voltage of inuput signal. And, what will happen if the input signal is less than
0.7V? The aunswer is, no signal will pass through diode because diode will be turned
off for any voltage less then 0.7 V. Then, how will one can rectifie a input signal less
which is then 0.7V? And due to this neccessity Precision rectifier comes into the
picture, precision rectifier is also known as Active rectifier preactive rectifier . Figure
35 shows the basic circuti diagram of an positive half wave precision rectifier.
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Precision Half Wave Rectifier
Figure 35
The precision half-wave rectifier can rectify signals with the peak value down to
few millivolts, unlike the conventional diode rectifiers. This is possible due to high
open loop gain of Op-Amp, because when D1 is turned off (or open) the Op-Amp in
figure 35 will behave as open-loop Op-Amp and what ever voltage is coming
through input terminal will multiply with open-loop gain (AOL) ,thus the minimum
value of input voltage require to turn on the diode D1 will be:
VIN (MIN) = 0.7/ AOL
Where, V IN(MIN) = minimum value of input to turn on the diode, for which the
voltage at anode (positive terminal) is 0.7V ,
If,
AOL = 105
V IN (MIN) = 0.7/ 105
V IN (MIN) = 7 x10-6 = 7μV
This will eliminate the effect of knee voltage. For instant, the knee voltage becomes
7μV instead of 0.7V. In fact the diode D1 acts as ideal diode. As VIN start increasing
in positive direction, the voltage at the diode D1’s anode also started increasing, and
for VIN = 7μV the voltage at D1’s anode will become 0.7V and diode D1 become
forward bias. When D1 become forward bias, it closes a feed back loop and the OpAmp works as voltage follower. Therefore, the output voltage VOUT is equal to VIN.
However when the input start increasing in negative direction the voltage at diode will
also increase until it will be equal to negative saturation voltage VEE, this reverse the
diode D1 and open the feed back loop. Therefore the output voltages become zero.
For a negative half wave rectifier, polarity of D1 should be changed. And hence for a
precision rectifier output will be
VP(OUT) = VP(IN)
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…….………. (27)
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Circuit diagram:
Figure 36
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Procedure:
•
To observe the Op-Amp as a precision rectifier,
1.
Connect the patch cord between socket ‘f1’ and ‘f2’ for positive half wave.
2.
Connect the on board function generator to input socket ‘IN6’, and then set the
input voltage VIN to 2V and observe the input at CH II of Oscilloscope.
3.
Connect the Oscilloscope CH I at tp15 and ground ‘Gnd’.
4.
Observe and note the output voltage of positive half wave.
5.
Verify the measure output is equal to calculated output by eq.27.
6.
Decrease the input voltage up to 0.5V, observe the output voltage.
7.
Connect the patch cord between socket ‘f1’ and ‘f3’ for negative half wave.
8.
Connect the on board function generator to input socket ‘IN6’, and then set the
input voltage VIN to 2V and observe the input at CH II of Oscilloscope.
9.
Connect the Oscilloscope CH I at tp15 and ground ‘Gnd’.
10.
Observe and note the output voltage of negative half wave.
11.
Decrease the input voltage up to 0.5V, observe the output voltage.
Note: The circuitry work better above 0.3V, lower then this voltage the output
will come but it will be distorted and noisy.
Observation diagram:
1.
For Positive Half Wave Precision Rectifier:
2.
For Negative Half Wave Precision Rectifier:
Conclusion: The rectified signal will appear even when the input voltage is less then
diode’s threshold voltage.
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Experiment 10
Objective:
To study and observe Op-Amp as active second order High Pass Filter.
Equipments Needed:
1.
Experiment board, Scientech 2323.
2.
Oscilloscope,
3.
Frequency counter,
4.
Multi-meter,
5.
2mm patch cords.
Filter:
A network design to attenuate certain frequency but passes another frequency without
attenuation is called filter. A filter circuit thus posses at least one pass band, which is
a band of frequency in which the output is approximately equal to the input
(attenuation is zero) and an attenuation band in which output is zero (attenuation is
infinite). The frequencies which separate the various pass and attenuation band are
called the cut-off frequencies or the frequency at which output becomes 0.707 of input
is called cut-off frequency. Thus a filter can define as a device that passes electric
signals at certain frequencies or frequency ranges while preventing the passage of
others. A Filter do not ideally transmit all the signal under the pass band with out
attenuation and complete suppress the signal in attenuation (or, stop band) with a
sharp cut-off profile due to absorption, reflection and other losses, this results as loss
of signal power.
Filters are termed as active or passive according to their component characteristics.
Passive filters are mainly network using inductors (L), resistors (R) and capacitors
(C). They are then called LRC filters. Passive filters consist of impedance
arrangement in series and parallel, two basic arrangements are T and ∏ section are
most commonly used.
Active filters are circuits that use an operational amplifier (Op-Amp) as the active
device in combination with some resistors and capacitors to provide an LRC-like filter
performance at low frequencies. However, the active filter requires high grade
technology for selected component and achieves the desired characteristics and
control. There are two principal reasons for the use of active filters. The first is that
the amplifier powering the filter can be used to shape the filter's response, e.g., how
quickly and how steeply it moves from its passband into its stopband. (To do this
passively, one must use inductors, which tend to pick up surrounding electromagnetic
signals and are often quite physically large.) The second is that the amplifier powering
the filter can be used to buffer the filter from the electronic components it drives. This
is often necessary so that they do not affect the filter's actions. Filter may be classified
according to different philosophies.
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Identifying their frequency characteristics, the filter differentiated as:
a.
Low pass filter (LPF)
b. High pass filter (HPF)
c.
Band pass filter (BPF)
d. Band stop filter (BSF)
High Pass Filter: It is a filter that passes high frequencies well, but attenuates (or
reduces) frequencies lower than the cutoff frequency. The actual amount of
attenuation for each frequency varies from filter to filter. It is sometimes called a lowcut filter; the terms bass-cut filter or rumble filter are also used in audio applications.
A high-pass filter is the opposite of a low-pass filter, and a bandpass filter is a
combination of a high-pass and a low-pass. It is useful as a filter to block any
unwanted low frequency components of a complex signal while passing the higher
frequencies. Of course, the meanings of 'low' and 'high' frequencies are relative to the
cutoff frequency chosen by the filter designer.
Idea frequency response of High pass filter
Figure 37
The simplest electronic high-pass filter consists of a capacitor in series with the signal
path in conjunction with a resistor in parallel with the signal path. The resistance
times the capacitance (R×C) is the time constant (τ); it is inversely proportional to the
cutoff frequency, at which the output power is half the input (−3 dB):
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RC combination for High pass filter
Figure 38
A second order high pass filter consists of two RC combinations with an Op-Amp’s
Non-inverting configuration.
High pass filter
Figure 39
The Low cutoff frequency of this configuration is
…………….(28)
Where gain of the Second order filter is
AF = 1.586
….………. (29)
= pass band gain for the second order Filter.
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Real frequency response of High pass filter
Figure 40
Such a filter could be used to direct high frequencies to a tweeter speaker while
blocking bass signals which could interfere with or damage the speaker. A low-pass
filter, using a coil instead of a capacitor, could simultaneously be used to direct low
frequencies to the woofer. High-pass and low-pass filters are also used in digital
image processing to perform transformations in the frequency domain. Most highpass filters have zero gain (-inf dB) at DC. Such a high-pass filter with very low
cutoff frequency can be used to block DC from a signal that is undesired in that signal
(and pass nearly everything else). These are sometimes called DC blocking filters.
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Circuit diagram:
Figure 41
Procedure:
•
To observe working of a second order active High Pass filter.
1.
Calculate the value of resistance for cut-off frequency equal to 2 KHz by using
given capacitance value in eq.28.
2.
Connect the probes of multi-meter at tp16 and tp17 and rotate the dual
Potentiometer P9 till the value of Potentiometer is equal to the calculated R,
3.
Connect the on board function generator to input socket ‘IN7’, and then set the
input voltage VIN to 100 Hz ,1VP and observe the amplitude of input at CH II of
Oscilloscope.
4.
Now connect the input at Frequency counter to read exact frequency.
5.
Connect the Oscilloscope CH I at tp18 and ground ‘Gnd’.
6.
Increase the frequency and note the output amplitude with the increment in the
frequency
7.
Voltage gain for second order filter is 1.586. So the output will be equal to
VOUT = 1.586 x VIN.
8.
Note the value of frequency for which there is 3db gain, this frequency is known
as cut off frequency, or the frequency at which, output voltage
VOUT = 0.707 x 1.586 x VIN; VOUT = 1.121 x VIN.
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9.
Plot the frequency response plot of output.
10.
Determine the difference between measured and calculated cut-off frequencies.
11.
Calculate the value of resistance for any cut-off frequency between 1 KHz to
10 KHz by using given capacitance value.
12.
Repeat the above step form 2 to 10 for new cut-off frequency.
Calculations:
1.
Cut-off frequency fL = 1/2πRC =
2.
Value of R =
3.
Pass band gain of Low pass filter AF = 1 + RF / R1 = 1.586
4.
Gain at 3 db frequency fH = 0.707 x AF ;VOUT = 0.707 x 1.586 x VIN
5.
Roll off rate = −40db/decade
Observation Table:
S. No.
Input
frequency
(Hz)
1
100
2
200
3
500
4
1K
5
5K
6
10 K
7
15 K
8
20 K
9
25 K
10
30 K
11
35 K
12
40 K
13
45 K
14
VOUT
50 K
Output amplitude Vs. frequency
Conclusion:
1.
The frequency response plot of the output amplitude is same as of shown in
Figure 40
2.
A very small difference between calculated and measured frequency.
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Experiment 11
Objective:
To study and observe Op-Amp as active second order Low Pass Filter.
Equipments Needed:
1.
Experiment board, Scientech 2323.
2.
Oscilloscope,
3.
Frequency counter
4.
Multi-meter,
5.
2 mm patch cords.
Low Pass Filter:
This filter passes low frequency but attenates (or reduces) frequencies higher than the
cut-off frequecy. The actual amount of attenuation for each frequency varies from
filter to filter. It is sometimes called a high-cut filter, or treble cut filter when used in
audio applications. An ideal low-pass filter completely eliminates all frequencies
above the cut-off frequecy while passing those below unchanged. The transition
region present in practical filters does not exist.
Idea Frequency Response of Low Pass Filter
Figure 42
However, this filter is not realizable for practical, real signals because the sinc
function extends to infinity. Hence there will be some tranisiton time in low pass
filter.
One simple electrical circuit that will serve as a low-pass filter consists of a resistor
in series with a load, and a capactor in parallel with the load. The capacitor exhibits
reactance, and blocks low-frequency signals, causing them to go through the load
instead.
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RC Combination for Low Pass Filter
Figure 43
At higher frequencies the reactance drops, and the capacitor effectively functions as a
short circuit. The combination of resistance and capacitance gives you the time
constant of the filter τ = RC (represented by the Greek letter tau). The break
frequency, also called the turnover frequency or cutoff frequency (in hertz), is
determined by the time constant: The Low cutoff frequency of this configuration is
……………..(30)
A second order Low pass filter consists of two RC combinations with an Op-Amp’s
Non-inverting configuration.
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Low pass filter
Figure 44
Where gain of the Second order filter is
AF = 1.586
…………. (31)
= pass band gain for the second order Filter.
Output Amplitude Vs. Frequency
Figure 45
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Circuit diagram:
Figure 46
Procedure:
•
1.
2.
3.
4.
5.
6.
7.
8.
To observe the working of a second order active Low pass filter
Calculate the value of resistance for cut-off frequency equal to 2 KHz by using
given capacitance value as in eq. 30.
Connect the probes of multi-meter at socket ‘IN8’ and tp19 and rotate the dual
Potentiometer P10 till the value of Potentiometer is equal to the calculated R,
Connect the on board function generator to input socket ‘IN8’, and then set the
input voltage VIN to 100 Hz ,1VP and observe the input amplitude at CH II of
Oscilloscope.
Now connect the input at Frequency counter to read exact frequency.
Connect the Oscilloscope CH I at tp20 and ground ‘Gnd’.
Increase the frequency and note the output amplitude with the increment in the
frequency
Voltage gain for second order filter is 1.586 so the output will be equal to VOUT =
1.586 x VIN.
Note the value of frequency for which there is 3db gain, this frequency is known
as cut off frequency, or the frequency at which, output voltage VOUT = 0.707 x
1.586 x VIN; VOUT = 1.121 x VIN.
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9.
Plot the frequency response plot of output.
10.
Determine the difference between measured and calculated cut-off frequencies.
11.
Calculate the value of resistance for any cut-off frequency between 1 KHz to
10 KHz by using given capacitance value.
12.
Repeat the above step form 2 to 9 for new cut-off frequency.
Calculations:
1.
Cut-off frequency fH = 1/2πRC =
2.
Value of R
3.
Pass band gain of Low pass filter AF = 1 + RF / R1 = 1.586
4.
Gain at 3 db frequency fH = 0.707 * AF ;VOUT = 0.707 x 1.586 x VIN
5.
Roll off rate = −40db/decade
=
Observation table:
S. No.
Input
frequency
(Hz)
1
100
2
200
3
500
4
1K
5
5K
6
10 K
7
15 K
8
20 K
9
25 K
10
30 K
11
35 K
12
40 K
13
45 K
14
50 K
VOUT
Conclusion:
1.
The frequency response plot of the output amplitude is same as of shown in
figure 45
2.
A very small difference between calculated and measured cut-off frequency
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Experiment 12
Objective:
To study and observe Op-Amp as active second order Band Pass Filter.
Equipments Needed:
1.
Experiment board, Scientech 2323.
2.
Oscilloscope,
3.
Multi-meter,
4.
Frequency counter
5.
2 mm patch cords.
Band Pass Filter:
Band pass filter are designed mathematically to respond to design frequencies while
rejecting all other out of band frequencies. A band pass filter can be designed to filter
a particular band, or spread, or frequencies from a wider range of mixed signals by
combining the properties of low pass and high pass filter. The series combination of
these two filter only allow passage of those frequency which are neither too high nor
two low.
Idea frequency response of Band pass filter
Figure 47
An ideal filter would have a completely flat pass-band (with no gain and attenuation
through out) and would completely attenuate all frequency outside pass-band. In
practice, no band-pass filter is ideal. The filter doesn’t attenuate all frequencies
outside the desired frequency range completely; in particular there is region just
outside the intended pass-band where frequencies are attenuated, but not rejected.
This is known as the filter roll-off, and is usually expressed in dB of attenuation per
decade of frequency.
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Second order Band Pass Filter Frequency Response
Figure 48
Generally the design of a filter seeks to make the roll-off as narrow as possible,
however as the roll-off is made narrower, the pass band is no longer flat; it begins to
ripple. This effect is particularly pronounced at the edge of the pass-band, an effect
known as Gibbs phenomenon. Between the lower cut-off and fL higher cut-off fH of a
frequency band is the resonant frequency, at which the gain of the filter is maximum.
The bandwidth of the filter is simply the different between fL and fH. A Wide Bandpass filter is formed by cascading a High pass filter and Low pass filter.
If the High-pass filter and Low-pass filter are of the first order then the Band-pass
filter will have a roll off rate of -20db/decade.
First order Band pass filter
Figure 49
If the High-pass filter and Low-pass filter are of the first order then the Band-pass
filter will have a roll off rate of -40db/decade.
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Second order Band pass filter
Figure 50
A01
= Pass band gain of High pass section
= 1 + RF / R
fL
= Low cut off frequency
= 1/2 π R1C1
A02
…………. (33)
= Pass band gain of Low pass section
= 1 + RF / R
fH
…………. (32)
…………. (34)
= High cut off frequency
= 1/2 π R2C2
…………. (35)
The voltage gain magnitude of wide band pass filter is the product of gains of low
pass sections (ALP) and high pass section (AHP)
Where the
Total Band pass gain A0 = A01 x A02
…………. (36)
What if we take the value of resistance and capacitance same for both of the high pass
and low pass circuit? Will they show no output? No, when we take value of resistance
and capacitance equal for RC combination of both low and high pass filter the, then
the ratio of higher and lower cut-off frequency become four.
i.e.
fH = 4.fL
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…………. (37)
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Circuit diagram:
.
Figure 51
Procedure:
•
To observe the working of a second order active Band pass filter
1.
Calculate the value of resistance for Lower cut-off frequency equal to 2 KHz
and Higher cut-off equal to 8 KHz by using given capacitance value.
2.
Use the eq.33 & eq.35 respectively.
3.
Connect the probes of multi-meter at tp16 and tp17 and rotate the dual
Potentiometer P9 till the value of Potentiometer is equal to the calculated
resistance, for Low cut-off frequency.
4.
Connect the probes of multi-meter at socket ‘IN8’ and tp19 and rotate the dual
Potentiometer P10 till the value of Potentiometer is equal to the calculated
resistance, for High cut-off frequency.
5.
Connect the on board function generator to input socket ‘IN7’, and then set the
input voltage VIN to 100 Hz ,1VP and observe the input amplitude at CH II of
Oscilloscope.
6.
Now connect the input at Frequency counter to read exact frequency.
7.
Connect the patch cord between socket ‘TP18’ and socket ‘IN8’ to configure a
band pass filter.
8.
Connect the Oscilloscope CH I at TP 20 and ground ‘Gnd’.
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9.
Increase the frequency and note the output amplitude with the increment in the
frequency
10.
Voltage gain for second order Low pass and high pass filter will be 1.586 so the
output will be equal to; VOUT = 0.707 x 1.586 x 1.586 x VIN.
11.
Note the first frequency for which there is 3db gain, this frequency is known as
Lower cut off frequency, fL; or the frequency at which, output voltage VOUT =
1.778 x VIN.
12.
Increase the frequency and note the second 3db gain frequency, known as higher
cut-off frequency, fH.
13.
Plot the frequency response plot of output.
14.
Determine the difference between measured and calculated lower and higher
cut-off frequencies.
15.
Calculate the value of resistance for any cut-off frequency between 1 KHz to
10 KHz by using given capacitance value.
16.
Repeat the above step form 2 to13 for new cut-off frequencies.
Calculations:
1.
Higher cut-off frequency fH = 1/2π R1C1
2.
Lower cut-off frequency fL = 1/ 2 πR2C2
3.
If , R1C1 = R2C2 = RC
4.
Value of R1 =
5.
Value of R2 =
6.
Pass band gain of Low pass filter AFL = 1 + RF / R1 = 1.586
7.
Pass band gain of High pass filter AFH = 1 + RF / R1 = 1.586
8.
Total gain AF = AFL x AFH =
9.
Gain at 3 db frequency fH = 0.707 x AF ;VOUT = 0.707 x AF x VIN
10.
Roll off rate = −40db/decade
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Observation Table:
S. No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Input
frequency
(Hz)
100
200
500
1K
5K
10 K
15 K
20 K
25 K
30 K
35 K
40 K
45 K
50 K
VOUT
Observation Graph:
Output amplitude Vs. frequency
Figure 52
Conclusion:
1.
The frequency response plot of the output amplitude is same as of shown in
figure 52.
2.
A very small difference between calculated and measured frequencies.
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Experiment 13
Objective:
To study and observe Op-Amp as active Notch Filter
Equipments Needed:
1.
Experiment board, Scientech 2323.
2.
Oscilloscope,
3.
Frequency counter,
4.
2mm patch cords.
Notch Filter:
A passive notch filter using only resistors and capacitors is shown in figure 52. It is
actually two filters in parallel, the upper one comprising two resistors and capacitor is
the low pass filter and the lower one comprising two capacitors and a resistor is high
pass filter. The stop bands of both the filter are overlapping. This makes it useful to
reject a narrow band of frequency. The reason it is called “twin-T” should be obvious.
Twin T
Figure 53
The notch frequency occurs where the capacitive reactance equals the resistance (Xc
= R) and if the values are close, the attenuation can be very high and the notch
frequency virtually eliminated.
The frequency of minimum gain (Notch frequency) is f0 =
1
2πRC
…………. (38)
The largest problem with this filter is that the input resistance is low at high
frequencies, being approximately R/4. Also the insertion loss of the filter will depend
on the load that is connected to the output, so the resistors should be of much lower
value than the load for minimal loss. Also the passive twin-T network has a relatively
low figure of merit Q. To overcome all these short comings Op-Amp is used in the
circuit as voltage follower as shown in figure 54.
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Scientech 2323
Figure 54
The junction of R/2 and 2C, which is normally connected to ground, is bootstrapped
to the output of the follower. Because the output of the follower is very low
impedance, neither the depth nor the frequency of the notch change; however, the Q is
raised in proportion to the amount of signal fed back to R/2 and 2C. The frequency
response of the Notch filter if as shown in figure 54
Figure 55
Procedure:
•
To observe the working of a Notch filter
1.
Calculate the notch frequency for Notch Filter by using the given value of
resistance and capacitance in eq.38.
2.
Connect the on board function generator to input socket ‘IN9’, and then set the
input voltage VIN to 100 Hz ,1VP and observe the input amplitude at CH II of
Oscilloscope.
3.
Now connect the input signal at Frequency counter to read exact frequency.
4.
Connect the Oscilloscope CH I at TP 21 and ground ‘Gnd’.
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5.
Increase the frequency and note the output amplitude with the increment in the
frequency.
6.
Voltage gain for second order band pass filter will be 1 one so the output will be
equal to VOUT = VIN.
7.
Note the frequency at which the output voltage is zero or negligible.
8.
Plot the frequency response plot of output.
9.
Determine the difference between measured and calculated Notch frequency.
Circuit diagram:
Figure 56
Calculations:
Notch frequency fN = 1/2π RC
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Scientech 2323
Observation Table:
S. No.
Input Frequency (Hz)
1
100
2
200
3
500
4
1K(fL)
5
5K
6
10K
7
15K
8
20K
9
25K
10
30K
11
35K
12
40K
13
45K
14
50K
VOUT
Output amplitude Vs. frequency
Figure 57
Conclusion:
1.
The frequency response plot of the output amplitude is same as of shown in
figure 57
2.
A very small difference between calculated and measured frequencies.
3.
At the Notch frequency the output voltage is zero or very low in milli volts.
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Scientech 2323
Warranty
1.
We guarantee this product against all manufacturing defects for 24 months from
the date of sale by us or through our dealers.
2.
The guarantee will become void, if
a. The product is not operated as per the instruction given in the Learning
Material.
b. The agreed payment terms and other conditions of sale are not followed.
c. The customer resells the instrument to another party.
d. Any attempt is made to service and modify the instrument.
3.
The non-working of the product is to be communicated to us immediately giving
full details of the complaints and defects noticed specifically mentioning the
type, serial number of the product and date of purchase etc.
4.
The repair work will be carried out, provided the product is dispatched securely
packed and insured. The transportation charges shall be borne by the customer.
Hope you enjoyed the Scientech Experience.
List of Accessories
1.
Patch Cord 16” (Red) 2mm ......................................................................4 Nos.
2.
Patch Cord 16” (Black) 2mm....................................................................2 Nos.
3.
Mains Cord .............................................................................................. 1 No.
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